16 May 2014

Water bears, or tardigrades, are harmless microscopic
animals. Yet, despite their endearing bear-like appearance, tardigrades are the
hardest animals to kill on Earth. And boy, many have tried.Tardigrades are chubby eight-legged animals, no longer than the head of a pin, related to velvet worms and also arthropodes, a large family including insects, spiders and crustaceans. They can be found anywhere where there’s water, but they prefer to live in damp moss and lichens. These tough creatures can survive boiling temperatures up to
125˚C* and freezing temperatures so extreme (-272˚C!) they can only be
artificially created in a laboratory. They can also survive astonishing amounts
of radiation with no apparent damage to their DNA, extremely high pressures,
and, unlike any other earthly creature, tardigrades can hang out for a few
minutes in the vacuum of space and come back alive to tell the story.

So what’s their secret? Tardigrades have the amazing
ability to reversibly slow down their metabolism to nearly a halt (less than
0,01% of their normal metabolic rate) in response to a change in their
environment—a process called cryptobiosis. Other organisms can do it—nematodes,
rotifers, brine shrimp—but not nearly as spectacularly as tardigrades. It is estimated that they can lose up to 99% of their water content, and enter a so-called
‘tun’ stage that protects them against harsh environmental conditions. Yet, if
you rehydrate these tuns, the animals will quickly return to their normal
selves—moving about, growing and having babies, as you do when you’re a
tardigrade (watch movie below).

Scientists grow tardigrades in the lab (and sometimes in space) to
study cryptobiosis. Understanding how tardigrades survive extreme dehydration during
the tun stage could help developing better techniques for dry
preservation of biological material, for example.In a recent study, Marcus Frohme and colleagues from the Technical
University of Applied Sciences in Wildau (Germany) compared differences in gene expression between happy, dehydrating, tun stage and rehydrated tardigrades.
The idea was to search for the genes that are more, or less active in each of
these metabolic states, which could give some clues as to how the tardigrades’
cells cope with severe dehydration. The researchers grew four groups of animals in the lab under different conditions (from moist to dry) and then smashed them up to chemically extract mRNA molecules (copies of DNA that will be
translated into proteins) from their cells. They then sequenced and quantified these molecules, and finally analysed the huge amount of data
using a powerful computer software.The team found that in
the dehydration stages, genes involved in cell division and growth were less
active, but genes encoding for proteins that protect or repair cellular
components, such as heat-shock proteins, were highly expressed. These results confirm previous research, but some preliminary
data in Frohme's study also suggest that several genes involved in DNA repair are more active
in the rehydration stage than in the dehydration stage. The authors propose
that tardigrades adopt a dual strategy combining mechanisms of protection (during dehydrating stages) and recovery (during rehydration stages) to survive desiccation.